Radio frequency (RF) tissue ablation is a well-known technique, e.g., in electrosurgery and thermal therapy, for making thermal lesions in the vicinity of an uninsulated tip of an electrode due to tissue coagulation caused by resistive heating. Voltage applied to electrodes causes electrical current flow through tissue and heat production due to tissue electrical resistance (Joule heating). The electrode can be applied directly on superficial structures, surgically, endoscopically, laparascopically, or even via a transcatheter access such as a treatment for symptomatic cardiac arrhythmias. If the electrode is formed as a needle, then the electrode may be inserted interstitially, and guided by imaging.
In a monopolar mode, current flows between a small target electrode and a large counter-electrode placed further away from the target. Due to the difference in the sizes of the electrodes, current density and associated Joule heat production are much higher at the target than at the return electrode. In contrast, in a bipolar mode, high density current flows between two adjacent target electrodes. Joule heat production is confined to a small volume due to electrodes size and proximity.
Thermal treatment amounts to applying high density current for a sufficient time to cause elevated temperature and associated physiological changes, e.g., coagulation, at a volume of tissue. Monopolar current flows through a larger volume compared to bipolar current. Consequently, monopolar Joule heating has a deeper penetration compared to bipolar heating, where the heat is confined to a small volume at the target electrodes
Electrosurgical apparatus is known that provides an option of selecting and switching between pure monopolar mode and pure bipolar mode. For example, U.S. Pat. No. 6,837,884 to Woloszko describes electrosurgical apparatus and methods for ablating, coagulating, shrinking, stiffening, or other treatment of a target tissue of a patient. The apparatus includes an electrosurgical probe, and an introducer needle adapted for passing through the distal end of the probe. In some embodiments, the electrosurgical system may include a dispersive return electrode for switching between bipolar and monopolar modes.